In most cellular wireless networks, it is important that base station transmit and receive frequencies are set with a high degree of accuracy. This is even more important with some of the newer airlink technologies such as Orthogonal Frequency Division Multiplexing (OFDM). In an OFDM system the absolute frequency for the signal transmitted from a base station has to meet very tight tolerances. Typical tolerances are of the order of 0.05 Parts Per Million (PPM). Additionally, in some wireless systems, like 4G wireless solutions based on OFDM, it is advantageous for all the base stations in a network to share a common timing reference so that the signals transmitted from the base stations are aligned in time.
There are many solutions for obtaining a long-term high accuracy transmit frequency. Approaches include locking the transmit frequency to a reference frequency provided by an atomic clock, using a reference frequency signal derived from the backhaul connections (e.g. deriving a frequency reference from a T1, E1 or fiber optic cable), or using a frequency reference provided by a Global Positioning System (GPS) receiver. Furthermore, each of these approaches provides a high accuracy timing reference.
The above solutions for obtaining a high accuracy frequency reference and timing reference are suitable for traditional cellular base stations that provide service to a relatively large geographical area, with GPS being the preferred option. These base stations are commonly referred to as macro base stations. The cost of a GPS receiver is relatively low compared to the overall cost of the macro base station. The backhaul approach is also feasible with network operators for their macro base stations. If a frequency reference or timing reference signal is derived from a backhaul connection, the operator of a macro base station can ensure that the signals on the backhaul are locked to a high accuracy frequency and time reference in the first place.
While prior art approaches to coordination and synchronization of base stations in a wireless network are adequate for existing macro base stations, they are not adequate for other types of wireless base stations. The market is shifting to lower cost base stations, commonly referred to as micro and pico base stations, as well as base stations intended for home usage, known as femto base stations. The price points for femto base stations preclude the use of a GPS receiver or atomic clock as a high accuracy frequency reference. Additionally, femto cells are typically installed indoors where a GPS receiver cannot receive a signal from the GPS satellite system that is required to provide the high accuracy frequency reference. The femto base station will typically be connected to a DSL modem or a cable modem over a standard 100BaseT Ethernet connection that cannot provide a sufficiently high accuracy frequency or timing reference. The IEEE 1588 standard for precision clock synchronization over a local area network is another option, but the achievable accuracy is severely limited when this technology is used with femto cells and other low cost wireless base stations connected to the core network over a DSL line or cable modem line. Ovenized oscillators are often utilized in current designs. These may be suitable as a long-term frequency reference, but are still relatively expensive and require calibration. A temperature compensated oscillator could also be used, but the long term frequency stability of such a solution is not sufficient to maintain long term tolerances of 0.05 PPM.
There is a need in present and future wireless networks to provide low-cost, high accuracy frequency and timing references to femto and other base stations in the network. Furthermore, there is a need for wireless networks to be able to update and coordinate the frequency and timing of signals from multiple base stations in the wireless network. There is also a need for providing frequency and timing references to femto and other base stations that do not drift over time like some prior art approaches.